USE 1 Completed Action Before Something in the Future

The Future Perfect expresses the idea that something will occur
before another action in the future. It can also show that something
will happen before a specific time in the future.

Examples:

By the time he gets home, she is going to have cleaned the entire house.

I am not going to have finished this test by 3 o'clock.

Will she have learned enough Chinese to communicate before she moves to Beijing?

Sam is probably going to have completed the proposal by the time he leaves this afternoon.

By next November, I will have received my promotion.

By the time I finish this course, I will have taken ten tests.

How many countries are you going to have visited by the time you turn 50?

Notice in the examples above that the reference points (marked in italics) are in Simple Present rather than Simple Future. This is because the interruptions are in time clauses, and you cannot use future tenses in time clauses.

USE 2 Duration Before Something in the Future (Non-Continuous Verbs)

With Non-Continuous Verbs and some non-continuous uses of Mixed Verbs, we use the Future Perfect to show that something will continue up until another action in the future.

Examples:

I will have been in London for six months by the time I leave.

By Monday, Susan is going to have had my book for a week.

Although the above use of Future Perfect is normally limited to
Non-Continuous Verbs and non-continuous uses of Mixed Verbs, the words
"live," "work," "teach," and "study" are sometimes used in this way even
though they are NOT Non-Continuous Verbs.

REMEMBER No Future in Time Clauses

Like all future forms, the Future Perfect cannot be used in clauses
beginning with time expressions such as: when, while, before, after, by
the time, as soon as, if, unless, etc. Instead of Future Perfect, Present Perfect is used.

Examples:

I am going to see a movie when I will have finished my homework.Not Correct